How to Use the Heating & Cooling Load Calculator
Before sizing a furnace, heat pump, or AC unit, you need to know roughly how much heat is escaping through — or entering — the building envelope. This calculator estimates the exterior wall area from your floor area (assuming a roughly square footprint) and ceiling height, then applies wall and window U-factors along with your window-to-wall ratio to compute the conduction load.
The formula is load (BTU/hr) = (net wall area × wall U-factor + window area × window U-factor) × design temperature difference. The wall U-factor presets follow representative IECC/ASHRAE 90.1 insulation tiers (low, code-minimum, high-performance), and window U-factors follow typical NFRC-rated glazing types.
This is a simplified conduction-only estimate — it does not include infiltration, ventilation, internal heat gains from people/lighting/equipment, or solar heat gain through glazing. For actual equipment sizing or a permit-ready design, have a mechanical engineer run a full Manual J (residential) or ASHRAE (commercial) load calculation before finalizing.
Frequently Asked Questions
This is a simplified envelope (wall and window) conduction load only — it excludes infiltration/ventilation, occupant and equipment heat gain, and solar gain. For actual equipment sizing, have a mechanical engineer run a full load calculation such as Manual J or ASHRAE methods.
For heating, subtract your local winter design outdoor temperature from your indoor setpoint (typically 68-70°F). For cooling, subtract your indoor setpoint (typically 75-78°F) from your local summer design outdoor temperature. Local design temperatures are published in ASHRAE climate design data.